Styrene, also known, as vinyl benzene, is an aromatic compound that is produced in industrial quantities from ethylbenzene. The most common method of styrene production comprises the dehydrogenation of ethylbenzene, which produces a crude product of styrene monomer and unreacted ethylbenzene and hydrogen. Polystyrene is an aromatic polymer produced from the styrene monomer. Polystyrene is a widely used polymer found in insulation, packaging, and disposable cutlery.
Different types of polystyrene materials can include general-purpose polystyrene (GPPS), high impact polystyrene (HIPS), and transparent impact polystyrene (TIPS). Many conditions affect the properties of the resulting product, including processing time, temperature, pressure, purity of the monomer feedstock, and the presence of additives or other compounds. These and other processing conditions alter the physical and chemical properties of the polystyrene product, affecting the suitability for a desired use.
Foamed polystyrene offers the advantages of low cost and high structural strength for its density. A typical polystyrene foam also has a relatively high impact resistance and possesses excellent electrical and thermal insulation characteristics. Foamed polystyrene is useful in a variety of applications such as insulation, packaging, coolers, food packaging, decorative pieces, and dunnage. Additionally, polystyrene foams are commonly classified into three general categories: low density, medium density, and high density. Low density polystyrene foams usually have a density of from about 1 to about 3 lb/ft3 whereas medium density foams may have a density ranging from about 4 to about 19 lb/ft3 and high density foams often have a density ranging from 20 to about 30 lb/ft3.
The two main types of polystyrene foam are extruded polystyrene (XPS) foam and expanded polystyrene (EPS) foam. Extruded polystyrene foam is typically formed by mixing polystyrene with additives and blowing agents into an extruder that heats the mixture. The mixture is then extruded, foamed to the desired shape, and cooled. Expanded polystyrene foam is typically formed by expanding solid polystyrene beads containing a blowing agent such as pentane with steam or hot gas. These pre-expanded beads may later be molded into the desired shape and expanded again with steam or hot gas to fuse the beads together.
In the production of foamed polystyrene, it is common to utilize blowing agents such as methyl chloride, ethyl chloride, chlorocarbons, fluorocarbons (including HFCs) and chlorofluorocarbons (CFCs). However, such blowing agents have been heavily regulated due to potential environmental impact. Many of these traditional and current physical blowing agents are halogenated compounds, which demonstrate a high solubility in polar polymers. An ongoing trend in foaming process development is to find environmentally benign chemicals as blowing agents. Some foaming processes have been using carbon dioxide (CO2) as the blowing agent or co-blowing agent. The advantages of using CO2 include low cost, minimal environmental impact, and eliminating potential fire hazards. It has therefore been desirable to use carbon dioxide as a blowing agent from both environmental and economic standpoints.
However, carbon dioxide has presented problems when used as a blowing agent. Carbon dioxide has been found to have a relatively low solubility in styrenic polymer melts. For example, the solubility of CO2 in polystyrene is only ca. 4 wt % at 6.5 MPa and 373 K, as measured by Yoshiyuki Sato et. al. (Journal of Supercritical Fluids 2001, 19, 187-198.). The low solubility results in high extrusion pressures, which increases costs and reduces quality. The low solubility also results in a higher density product. It would be desirable to obtain a polystyrene product having a high carbon dioxide solubility in order to reduce costs and increase product quality.
Furthermore, carbon dioxide has relatively higher vapor pressure and diffusivity, compared to halogenated blowing agents. In the extrusion foaming process using CO2 as the blowing agent, the melt strength of polystyrene is often inadequate, which leads to immature bubble breakage/coalescence, non-uniform cell morphology, and excessive open cell content. It would be desirable to obtain a polystyrene resin having improved melt strength in order to perform well in foaming processes.